T-joins in Strongly Connected Hypergraphs G. Calinescu ∗ Abstract Given an edge-weighted undirected hypergraph K = (VK , EK ) and an even-sized set of vertices R ⊆ VK , a T-cut is a partition of VK into two parts Q and Q¯ := VK \ Q such that |Q ∩ R| is odd. A T-join in K for R is a set of hyperedges M ⊆ EK such that for every T-cut (Q, Q¯) there is a hyperedge e ∈ M intersecting both Q and Q¯. A directed hypergraph has for every hyperedge exactly one vertex, called the head, and several vertices, that are tails. A directed hypergraph is strongly connected if there exists at least one directed path between any two vertices of the hypergraph, where a directed path is defined to be a sequence of vertices and hyperedges for which each hyperedge has as one of its tails the vertex preceding it and as its head the vertex following it in the sequence. Orienting an undirected hypergraph means choosing a head for each hyperedge. We prove that every edge-weighted undirected hypergraph that admits a strongly connected orientation has a T-Join of total weight at most 7/8 times the total weight of all the edges of the hypergraph, and sketch an improvement to 4/5. We also exhibit a series of example showing that one cannot improve the constant above to 2/3 − ǫ. keywords: weighted hypergraph, directed hypergraph, T-join, T-cut 1 Introduction Given an edge-weighted hypergraph K = (VK , EK) and an even-sized set of vertices R ⊆ VK , a T-cut is a partition of VK into two parts Q and Q¯ := VK \ Q such that |Q ∩ R| is odd. A T-join in K for R is a set of hyperedges M ⊆ EK such that for every T-cut (Q, Q¯) there is a hyperedge e ∈ M intersecting both Q and Q¯; such an hyperedge is said to cross the T-cut. See Figure 1 for intuition. A minimum-weight T-join can be computed in polynomial-time if K is a graph (Chapter 29 of [9]). The generalization of Minimum Weight Graph T-join to hypergraphs, which we call Hypergraph T-join, is however NP-hard (Section 2). A directed hypergraph has for every hyperedge exactly one vertex, called the tail, and several vertices, that are heads (to avoid having to rewrite most of the paper, we reverse the standard tail- head notation from the abstract above or from [5]; this reversal does not affect the results). A directed hypergraph is strongly connected if there exists at least one directed path between any two vertices ∗Department of Computer Science, Illinois Institute of Technology, Chicago, IL 60616, USA. [email protected]. Research supported in part by NSF grants CCF-0515088 and NeTS-0916743. Based on results from an extended abstract published in Springer LNCS 6520: Proc. 13th International Workshop on Approximation Algorithms for Combinatorial Optimization Problems, 67–80 (APPROX 2010). 1 y0 y1 y2 x u2 u1 z2 z0 z1 Figure 1: An example of a hypergraph T-join. R is given by the solid dark nodes. Another example appears later in Figure 3. of the hypergraph, where a directed path between vertices u and v is defined to be an alternating sequence of vertices and hyperedges, starting with u and ending with v and such that each hyperedge has as tail the vertex preceding it and as one of its heads the vertex following it in the sequence. Orienting an undirected hypergraph means choosing a tail for each hyperedge. [5] characterizes the undirected hypergraphs that admit strongly connected orientations. We study the supremum, over classes of hypergraphs and all possible R, of the minimum weight T-join divided by the weight of (all hyperedges in) the hypergraph. For (the class of) two-edge- connected graphs, this T-ratio is known (and not too hard to prove) to be 1/2 [4]. Let K =(VK , EK) be an undirected hypergraph. A path in a hypergraph consists of an alternating sequence of vertices and hyperedges for which each hyperedge contains the two vertices which pre- cede and follow it in the sequence. Two vertices of K are in the same connected component if there exists a path with the two vertices as endpoints. It is easy to see that M is a T-join in K for R if and only if every connected component of the hypergraph (VK, M) has an even number of vertices of R. A hypergraph is two-edge-connected if there exist two hyperedge-disjoint paths between any two vertices. Note that Menger’s theorem holds for hypergraphs and a hypergraph is two-edge-connected if and only if the removal of any single hyperedge does not disconnect the graph. It is not true [5] that every two-edge-connected hypergraph admits a strongly connected orientation (for graphs, this property holds and is known since at least 1939 [8]). However it is easy to prove that the undirected version of a strongly connected hypergraph is two-edge-connected. In this paper we prove that for the class of edge-weighted undirected hypergraph that admit a strongly connected orientation, the T-ratio is at most 7/8, and sketch an improvement to 4/5. We also exhibit a series of example showing that the T-ratio is at least 2/3. For the class of edge-weighted undirected hypergraphs that are two-edge-connected, we show that the T-ratio is 1. The T-ratio for hypergraphs that admit strongly connected orientations was used by [2] to improve the approximation ratio for a power assignment problem, described below. A submitted journal ver- sion has an improved ratio (for the same algorithm) without using the T-ratio. In the first analysis of this algorithm, the T-ratio plays a similar role to the often used Steiner ratio [10, 1] for Steiner Tree. This paper still uses power assignment notation in addition to hypergraphs. 2 1.1 Power Assignment and Min-Power Strong Connectivity Power Assignment problems take as input a directed simple graph G = (V, E) and a cost function c : E → R+. The power of a vertex u in a directed spanning simple subgraph H of G is given by pH (u) = maxuv∈E(H) c(uv), and corresponds to the energy consumption required for wireless node u to transmit to all nodes v with uv ∈ E(H). The power (or total power) of H is given by p(H)= u∈V pH (u). The studyP of Power Assignment was started by Chen and Huang [3], which consider, as we do, the case when E is bidirected, (that is, uv ∈ E if and only if vu ∈ E, and if weighted, the two edge have the same cost; this case was sometimes called “symmetric” or “undirected” in the literature) while H is required to be strongly connected. See also [7]. We call this problem MIN-POWER STRONG CONNECTIVITY. We use with the same name both the (bi)directed and the undirected version of G. Hypergraphs arise naturally for MIN-POWER STRONG CONNECTIVITY (more so than for Steiner Tree), see Figure 2 for intuition. y z u x v Figure 2: The arrows present a directed graph H. The rounded rectangles show an undirected hyper- graph, obtained from H, by having a hyperedge (a subset of vertices) consisting of a vertex v ∈ V and all u ∈ V with vu ∈ H, with the weight of a hyperedge typically being pH (v). Thus in this example, the weights w({x, y, z}) = max(c(xy),c(xz)) and w({u, v})= c(uv). 2 Preliminaries In directed graphs, we use arc to denote a directed edge. In a directed graph K, an incoming arbores- cence rooted at x ∈ V (K) is a spanning subgraph T of K such that the underlying undirected graph of T is a tree and every vertex of T other than x has exactly one outgoing arc in T . Given an arc xy, its undirected version is the undirected edge with endpoints x and y. An alternative definition of MIN-POWER STRONG CONNECTIVITY (how it was originally posed) is: we are given a simple undirected graph G = (V, E) and a cost function c : E → R+. A power assignment is a function p : V → R+, and it induces a simple directed graph H(p) on vertex set V given by xy being an arc of H(p) if and only if {x, y} ∈ E and p(x) ≥ c({x, y}). The problem is to minimize u∈V p(u) subject to H(p) being strongly connected. To see the equivalence of the definition, givenP directed spanning subgraph H, define for each u ∈ V the power assignment p(u)= pH (u). While it may be already known, it is easy to check that Hypergraph T-join is indeed NP-hard, by a reduction from 4-D Matching (which asks if a 4-regular hypergraph K with V (K) a multiple of 4, 3 contains a perfect matching, that is, a set of disjoint hyperedges containing every vertex of the input hypergraph; see Garey and Johnson [6] problem SP1). It is easy to check that for R = V (K), a T-join of size at most |V (K)|/4 must be a perfect matching. For u ∈ V and r ∈{c(uv) | uv ∈ E}, let S(u,r) be the directed star with center u containing all the arcs uv with c(uv) ≤ r; note that r is the power of S. For a directed star S, let E(S) be its set of arcs and V (S) be its set of vertices.